It's not arbitrary at all. Born babies are sentient people. A pregnancy is not a person. 50% of all human pregnancies spontaneously abort. That's why your church won't baptize a pregnancy. That's why you can't get a social security number for your pregnancy. Once a baby is successfully born it's an autonomous person who gets to join society. Removing an early pregnancy isn't murdering a person. Not being born is not the same as a person being murdered. You cheapen life when you say things like that. However, the pregnant person IS a legal person with rights. Having a child is the most important decision any person can make and shouldn't be forced on anyone who isn't ready.